Integrated circuits, such as monolithic semiconducting devices, are formed by layering materials with different properties on top of each other to form devices such as resistors, capacitors, and transistors of different types into complex circuits. The different layers are processed in a variety of ways. For example, one layer may need to have features etched into it, where the etched features form voids that extend down into the layer. Other layers may need to be deposited into such etched voids. Often, the one layer that is deposited into the voids of another layer may need to be removed from the top of the layer in which the voids are formed, so that the second layer is only within the voids.
Because of these and other specialized requirements for the physical shape of the different layers used in integrated circuit fabrication, specialized instruments have been developed to help determine the topography and thickness of the various layers that are formed. For example, the topography of a layer of transparent material, such as an oxide, can be determined from instruments that physically contact the surface of the transparent layer, such as mechanical profilers or atomic force microscopes. The thickness of transparent layers can be determined by the use of spectroreflectometers, or ellipsometers.
If the layer is formed of a non transparent or opaque material such as a metal, then in addition to mechanical profilers, other methods can be used to determine the topography of the layer, such as an interferometric profiler. Unfortunately, interferometric profilers cannot be used to determine the topography of transparent layers, or layers such as those briefly described above where a transparent material is disposed alongside an opaque material in an array, because the phase shifts dependent on the thickness of the transparent material confounds the reflected readings received back by the interferometric profiler. Further, a spectroreflectometer or an ellipsometer cannot be used to measure the topography of either type of layer, or to measure the thickness of an opaque layer. Thus, the instruments that have been developed are somewhat limited in their application, but work well when the properties of just an opaque layer or just a transparent layer are to be determined.
Unfortunately, there are more and more applications of mixed layers such as those described above, where a material such as copper is deposited into the voids that have been etched into a material such as silicon dioxide, and then the copper is thinned such as in a chemical mechanical planarization process down to the level of the upper surface of the silicon dioxide. When such a structure is formed, an array of both opaque sections and transparent sections is formed in the top surface of the compound layer. This array is very difficult to characterize without the use of an instrument that makes physical contact with the surface, such as the mechanical profilers and atomic force microscopes. However, it is often desirable to not make any contact with the surface of the array.
There is a need, therefore, for a system by which both the topography of the array surface and also the thickness of the transparent material portions of the array can be determined without making physical contact with the array layer surface.